Oxidative removal of aqueous metal-complexed cyanide under acidic conditions

ABSTRACT

The oxidation of metal-complexed cyanide under acid conditions using oxygen as the oxidizing agent can be performed effectively with certain metal chelates as catalysts. Especially effective chelates are metal phthalocyanines, particularly where the metal is vanadium or a member of the iron group metals. The oxidation can be effected homogeneously using water soluble metal chelates, or can be performed heterogeneously, especially in a continuous fashion using a packed bed reactor, by using suitable water-insoluble metal chelates, especially when supported on appropriate carriers.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-pan of my copending application,Ser. No. 08/016,355, filed Feb. 11, 1993, all of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

It is only in relatively recent years that society has appreciated theimpact and consequences of the fact that we live in a closed ecologicalsystem. With an increase in human population and, perhaps moreimportantly, an increase in industrial activity the effects ofecological changes have become more apparent. One area which hasreceived a great deal of attention is that of water quality, which maybe the result of the belated recognition that not only is water of asuitable quality for human consumption a limited resource, but that goodwater quality is an important, if not critical, factor in the ecologicalchain. Consequently attention has turned not only to purification ofwater in local water supplies, but also to limiting the discharge ofmaterials into streams and aquifers generally.

The classes of noxious materials (pollutants) in aqueous discharges varyover an enormously broad spectrum. Among the inorganic pollutants thosetoxic to a broad spectrum of biological species are especiallydangerous. Although heavy metals such as lead, cadmium, and arsenicoften are the first culprits thought of, inorganic water soluble cyanideis in a comparably dangerous class because of the generally lowtolerance of life forms to cyanide.

The sources of cyanide are many and varied and include iron and steelmanufacturing, petroleum and coal pyrolysis processes, the photographic,chemicals, and pharmaceutical industries, precious metal mining andmetal finishing, including electroplating and galvanizing. For example,cyanide arises in iron and steel manufacture by reduction of carbonatein the presence of carbon and nitrogen. In power plants coal burning mayafford coke oven gas with a hydrogen cyanide concentration on the orderof 2 grams per liter. Cyanide solutions are an important component ofelectroplating and galvanizing, and wash water streams resulting frompost-coating treatment often contain significant quantities of cyanide.The widespread prevalence of cyanide in industrial effluents coupledwith their near universal toxicity to life has made it imperative tominimize cyanide concentration in aqueous streams.

It appears that the most prevalent methods of cyanide removal are basedon the oxidation of cyanide. See generally R. Gierzatowicz et at.,Effluent and Water Treatment Journal, 25, 26-31 (1986). Oxidation withchlorine or hypochlorite seems to be industrially the most commonlyemployed method. The first stage in this oxidation is the formation ofcyanogen chloride, ClCN, itself a rather toxic gas, but which ishydrolyzed at a high pH to the less toxic cyanate, CNO. Cyanate isitself hydrolyzed to carbon dioxide and ammonia at low pH, or is furtheroxidized to carbon dioxide and nitrogen. Another oxidative method usesperoxides such as hydrogen peroxide, Caro's acid, peracetic acid, and soon, as the oxidizing agent. The advantages of this approach vis a visthe chlorine or hypochlorite based process is the lack of toxicbyproducts and the formation of environmentally neutral species from theperoxides. A disadvantage is the long reaction times necessary foradequate oxidation. However, cupric ions supposedly act as catalysts forperoxide oxidation. Other oxidizing agents based on Mn(VII) and Cr(VI)also have been used.

More recently there has been described the oxidation of both free andcomplexed cyanide in aqueous streams by a mixture of sulfur dioxide oralkali/alkaline earth metal sulfites (including bisulfites andmetabisulfites) and air or water in the presence of a water-solublecopper(II) catalyst at a pH between 5 and 12; U.S. Pat. No. 4,537,686.[Although copper is designated as "Cu⁺ " in the issued patent, the factthat most cuprous salts are water insoluble and that Cu(I) is readilyoxidized strongly suggests that Cu(II) actually was used.] Using ratherhigh weight ratios of copper to cyanide on the order of about 0.25,final cyanide concentrations could be reduced to under 0.1 parts permillion. More recently Chen et al. (Paper 81c presented at the 1990AIChE Summer National Meeting, San Diego, Calif., Aug. 21, 1990)presented data on the oxidation with air of aqueous streams containingcyanide at 100 ppm using a soluble copper catalyst in conjunction withsodium sulfite at an optimum pH of 8 over activated carbon in a tricklebed reactor at normal pressure. Initially the copper/cyanide molar ratiowas about 0.25, but since copper(II) hydroxide precipitated on thecarbon surface, it was found that a copper/cyanide maintenance ratio ofabout 0.1 was quite adequate. Although the authors characterize theactivated carbon as a catalyst, this conclusion is far from clearaccording to the data. Thus, although the authors showed that use of abed of activated carbon leads to 99% removal of cyanide, beds of both amolecular sieve and glass beads were almost as effective in affordingabout 80% removal. The improved result with activated carbon couldreadily be attributed to adsorption (rather than oxidation) on the bedof activated carbon -- activated carbon is known to be an excellentadsorbent -- or to the differing extent of copper(II) deposition on thepacked beds and its dispersion on the bed materials, or to somecombination of the two.

A continuous method for the removal of cyanide using air or oxygen asthe oxidizing agent at ambient temperatures and pressures is highlydesirable. Although the foregoing references provide a start, muchremains before a commercially viable system is operative. In particular,it is often desirable that the catalyst either be heterogeneous, or ifhomogeneous readily separable, in order to avoid contamination of theeffluent by the catalyst itself as well as to minimize process costassociated with catalyst consumption. It also is desirable that thecatalyst be relatively insensitive to as large a class of contaminantslikely to accompany cyanide as is possible. The process should becapable of efficient operation at as low a pressure and temperature aspossible in order to minimize energy requirements. Finally, it isdesirable for such a process to oxidize the cyanide over a rather widerange of initial cyanide concentrations, and to have the capability ofoxidizing 90% or more of the cyanide present, most preferably to carbondioxide, and any combination of elemental nitrogen and ammonia (orammonium ion in acidic media).

U.S. Pat. No. 5,120,453 provides a process for the oxidation of cyanidein aqueous streams where the cyanide is present as the anion, CN⁻, andwhere the oxidation is performed under basic conditions. It may be notedthat the cited prior art also emphasizes cyanide oxidation under basicconditions, and it also may be noted in passing that basic conditionseither are a prerequisite to, or materially enhance the concentration ofcyanide ion, so that the correlation between prior art oxidation underbasic conditions and the presence of cyanide ion may be a fundamentalone rather than being fortuitous. The patentees of the last cited patentuse as catalysts a broad class of metal chelates which can be usedeither in a soluble or water-insoluble form to afford the opportunity ofeither a homogeneous or heterogeneous process. The oxidation productswere largely carbon dioxide and nitrogen along with varying amounts ofcyanate.

A more challenging task is the oxidation of metal complexed cyanideswhose stability constant is so high that they dissociate only to anegligible extent and therefore, for all practical purposes, furnish nomeasurable amount of free cyanide ion. U.S. Pat. No. 5,238,581 providedone type of solution to this task by ultraviolet irradiation of theaqueous complexed cyanide to promote its dissociation to free cyanidewhich, in turn, could be readily oxidized.

In Ser. No. 08/016,355 we described the unexpected oxidation ofinorganic cyanides under acidic conditions. In this application wedescribe the oxidation of metal complexed cyanides under similarconditions. It is well known that the cyanide ion complexes stronglywith many metals to form stable complexes, e.g., ferrocyanides, whosedissociation constant is so small that the attending minisculeconcentration of free cyanide ion is insufficient for any practicaloxidation. Thus, the cyanide port/on of these metal complex cyanides mayfor all practical purposes be oxidation resistant under basicconditions. However, these complexes are dissociated to some extent inacidic media (to form HCN). In the invention we describe within, whichutilizes oxidation under acidic conditions, these metal-complexedcyanides can be effectively and conveniently oxidized, in contrast tothe prior art. Thus our invention now opens the possibility of thedirect oxidation of many cyanide complexes such as result from miningand electroplating operations. This will be elaborated on more fullywithin.

Yet another incidental but significant benefit from oxidation of cyanideunder acidic conditions is that the products are CO₂, N₂, and ammoniumion, NH₄ ⁺. Under basic conditions the products are CO₂, N₂, andcyanate, NCO⁻. Although the cyanate anion formed under basic conditionsis relatively benign, nonetheless the ammonium ion formed under acidicconditions is environmentally far more preferable. Thus the overallresult is that the oxidation products formed under acidic conditions areenvironmentally more benign than those products formed under basicconditions. Consequently acid oxidation of metal-complexed cyanides isan environmental coup.

SUMMARY OF THE INVENTION

The purpose of this invention is to reduce the metal-complexed cyanideconcentration in aqueous streams using as mild an oxidant as possible,and preferably oxygen, in an economical process capable of treating amultitude of streams where the oxidation is performed under acidicconditions. An embodiment comprises oxidizing the water solublemetal-complexed cyanide by contacting the cyanide-laden stream with anoxidizing agent in the presence of a catalyst which is a metal chelateat a pH under 7. In a specific embodiment the chelate is aphthalocyanine of cobalt, vanadium, nickel, or iron. In a more specificembodiment the catalyst is a chelate of a sulfonated cobaltphthalocyanine dispersed on carbon and the oxidizing agent is oxygen. Inyet another embodiment the catalyst is a water-soluble polysulfonatedcobalt phthalocyanine. In a different embodiment the aqueous streamcontains the metal-complexed cyanide at a pH between about 1 to about 6.Other embodiments will be apparent from the ensuing description.

DESCRIPTION OF THE INVENTION

The need to remove cyanide from metal-complexed cyanides in variouswaste water streams has been elaborated upon above. Although variousmethods currently may be available, there remains a need for a processwhich is at once sufficiently flexible to be applicable to varyingconcentrations of the metal-complexed cyanide, to be capable of beingadaptable to both continuous and batch processes, to be catalytic innature, and to be capable of using oxygen as the oxidizing agent. Ofoverriding importance is the need for a process which oxidizes at least90% of the cyanide present in metal-complexed cyanides whosedissociation constant is so low that less than 1% of the complexedcyanide dissociates to free cyanide. We have found that a class of metalchelates previously used under basic conditions in the oxidation ofsulfides, especially mercaptans, also are effective in the oxidation ofmetal-complexed cyanides under acidic conditions. This observation wasrather unexpected, in view of the experience that sulfide oxidation bythe aforesaid metal chelates preferred base, the prior art teachings ofcyanide oxidation (by other catalysts) under basic conditions, the lowdegree of dissociation of the complexed cyanides, and the concern thatthe aforesaid metal chelates would leach under acidic conditions.Accordingly, it was not expected that these metal chelates would oxidizemetal-complexed cyanides effectively under the acidic conditionsdescribed within.

It needs to be explicitly understood and recognized that the permissiblecyanide level remaining after treatment of the water stream is variable.For example, the proposed standards for drinking water sets a level of0.2 ppm as the maximum permissible. If an electroplater of common metalsdischarges to a publicly owned treatment waterwork less than 38,000liters per day, the 4-day average of cyanide amenable to treatment isnot more than 2.7 ppm. For the same type facility discharging 38,000liters or more per day, the 4-day average can not exceed 1.0 ppm oftotal cyanide. From the foregoing it should be clear that a variety offinal cyanide levels will be found acceptable; no single standard may bestated.

Any aqueous stream containing at least one metal-complexed cyanide issuitable for use in this invention, which is to say that the nature ofthe water-soluble metal-complexed cyanide is largely immaterial, acondition which is in stark contrast to that for oxidation under basicconditions where the cyanide ion, CN⁻, appears to be a necessaryprerequisite for oxidation. It is well known that cyanide complexes withmany metals in several oxidation states to form a dazzling number andvariety of metal cyanide complexes. Such metal complexes may be found invarious compilations well known to and readily available to thepractitioner, and therefore need not be elaborated upon here. Exemplaryof such complexes are the well known iron complexes, such ashexacyanoferrate (II) and (III), gold complexes such as dicyanoaurate(I) and tetracyanoaurate (III), silver complexes as dicyanoargentate(I), chromium complexes such as dicyanochromate (III), nickel complexessuch as hexacyanonickelate (II), copper complexes as tricyanocuprate (I)and tetracyanocuprate (II). It must be recognized that the foregoing aremerely exemplary and representative, and many more complexes have beendocumented. Exemplary of the metals in the metal-complexed cyanides areiron, chromium, nickel, copper, gold, silver, cadmium, mercury, zinc,platinum, cobalt, molybdenum, manganese, vanadium and titanium, tomention but a few metals.

The advantages presented by the capability of our invention to oxidizemetal-complexed cyanides under acidic conditions may be seen moreclearly when comparing the present practice of cyanide removal fromstreams containing strongly complexed cyanide with the procedure ourinvention makes possible. Previously, aqueous streams containingmetal-complexed cyanides were treated with strong bases to precipitate acyanide-containing sludge, the sludge was collected and thereaftertransported to a central site. At the central site the sludge wasacidified, the generated HCN was trapped in a basic solution, and thebasic cyanide then was oxidized. In contrast, in the practice of ourinvention the aqueous stream containing metal-complexed cyanide needonly be acidified and the acidified stream oxidized directly, especiallywhere the acidified stream does not itself contain particulates orsludge.

Our invention is applicable most desirably to streams containing up toabout 500 parts per million cyanide in the metal-complexed cyanide,although it is preferably applicable to streams containing no more thanabout 100 ppm cyanide. Many streams contain cyanide in metal-complexedcyanide on the order of 5 ppm, and for these streams our invention isespecially effective. However, it needs to be clearly understood thatour invention may be applicable to streams containing as much as severalpercent cyanide in metal-complexed cyanide, although such streams may bean uncommon occurrence. Metal-complexed cyanide-laden aqueous streamsinclude waste streams from metal plating industries, from photographylaboratories and steel mills and streams from the mining industry.However, the nature of the metal-complexed cyanide-containing streamswhich can be treated by the process of our invention is not particularlycritical in any way, for metal-complexed cyanides generally are oxidizedby our process. Yet it also must be recognized that there isconsiderable diversity among the streams as to their source. Forexample, waste streams from mining generally may contain predominantlyiron-complexed metal cyanides, whereas waste streams from a platingplant probably will have mainly chromium-complexed cyanides.

The metal-complexed cyanides which are the subject of this invention arecharacterized by having high stability constants (i.e., small degree ofdissociation into cyanide) so that only a quite small fraction of thecomplexed cyanide becomes available as free cyanide ion. This can beexemplified by the hexacyanoferrate(II), Fe(CN)₆ ⁴⁻, whose dissociationconstant to Fe(CN)₅ ³⁻ +CN⁻ at 20° C. is about 10⁻⁸.3. This means thatat a 1 molar concentration of hexacyanoferrate (II) the cyanideconcentration is about 10⁴ molar, or about 0.002 percent of the totalcyanide present. As another example, where the hexacyanoferrate (II) ispresent at a concentration of about 1 weight percent (ca. 0.05 molar)the free cyanide concentration will be about 0.003 weight percent (ca.10⁻⁵ molar), or about 0.003 percent of the total cyanide present. Ingeneral, the metal-complexed cyanides of our invention are characterizedby furnishing at 20° C. under 1 percent of the total available cyanideas free cyanide, and generally afford at 20° C. less than 0.1 percent ofthe total available cyanide as free cyanide.

The key to our invention is our discovery that certain metal chelatesare effective in catalyzing the oxidation of inorganic cyanides,including metal-complexed cyanides, under acidic conditions by suchoxidizing agents as air itself. The metal chelates which act ascatalysts are known to the art as effective in catalyzing the oxidationof mercaptans contained in a sour petroleum distillate to disulfides.The metal chelates include the metal compounds oftetrapyridinoporphyrazine described in U.S. Pat. No. 3,980,582, e.g.,cobalt tetrapyridinoporphyrazine; porphyrin and metalloporphyrincatalysts as described in U.S. Pat. No. 2,966,453, e.g., vanadiumtetraphenylporphin carboxylate; corrinoid catalysts as described in U.S.Pat. No. 3,252,892, e.g., manganese corrin sulfonate; chelateorganometallic catalysts such as described in U.S. Pat. No. 2,918,426,e.g., the condensation product of an aminophenol and a metal of GroupVIII; and the metal phthalocyanines as described in U.S. Pat. No.4,290,913, etc. As stated in U.S. Pat. 4,290,913, metal phthalocyaninesare a preferred class of metal chelates.

The metal phthalocyanines which can be employed to catalyze theoxidation of mercaptans generally include magnesium phthalocyanine,titanium phthalocyanine, hafnium phthalocyanine, vanadiumphthalocyanine, tantalum phthalocyanine, molybdenum phthalocyanine,manganese phthalocyanine, iron phthalocyanine, cobalt phthalocyanine,platinum phthalocyanine, palladium phthalocyanine, copperphthalocyanine, silver phthalocyanine, zinc phthalocyanine, tinphthalocyanine, and the like. The iron-group (Group VIII metals)phthalocyanines and vanadium phthalocyanines are particularly preferred,and among the iron-group phthalocyanines cobalt phthalocyanine isespecially preferred. The ting substituted metal phthalocyanines aregenerally employed in preference to the unsubstituted metalphthalocyanine (see U.S. Pat. 4,290,913), with the sulfonated metalphthalocyanine being especially preferred, e.g., cobalt phthalocyaninemonosulfate, cobalt phthalocyanine disulfonate, etc. The sulfonatedderivatives may be prepared, for example, by reacting cobalt, vanadiumor other metal phthalocyanine with fuming sulfuric add. While thesulfonated derivatives are preferred, it is understood that otherderivatives, particularly the carboxylated derivatives, may be employed.The carboxylated derivatives are readily prepared by the action oftrichloroacetic acid on the metal phthalocyanine.

The degree of derivatization importantly affects the solubility of themetal chelates, such as the phthalocyanines, of this invention. Usingthe phthalocyanines as a specific example, monosulfonation affords achelate which still is water insoluble (under 0.1 weight percent) andwhich quite suitably can be dispersed on a catalyst support or carrierfor use in heterogeneous catalysis of cyanide in aqueous streams. On theother hand, polysulfonation up to 34 sulfonic acid residues perphthalocyanine affords a metal chelate which is water soluble and whichis readily adaptable for use as a homogeneous catalyst under aqueousreaction conditions. The soluble metal chelates could be used, forexample, in toxic waste storage ponds or in other storage facilities,especially in conjunction with aeration.

For use in a packed bed, heterogeneous catalytic operation the metalphthalocyanine catalyst can be adsorbed or impregnated on a solidadsorbent support in any conventional or otherwise convenient manner. Ingeneral, the support or carrier material in the form of spheres, pills,pellets, granules or other particles of uniform or irregular shape andsize is dipped, soaked, suspended or otherwise immersed in an aqueous oralcoholic solution and/or dispersion of the metal phthalocyaninecatalyst, where the aqueous or alcoholic solution and/or dispersion maybe sprayed onto, poured over, or otherwise contacted with the adsorbentsupport. In any case, the aqueous solution and/or dispersion isseparated, and the resulting composite is allowed to dry under ambienttemperature conditions, or dried at an elevated temperature in an ovenor in a flow of hot gases, or in any other suitable manner. In general,up to about 25 weight percent metal phthalocyanine can be adsorbed onthe solid adsorbent support or carrier material and still form a stablecatalytic composite. A lesser amount in the range from about 0.1 toabout 10 weight percent generally forms a suitably active catalyticcomposite, although the activity advantage derived from metalphthalocyanine concentrations in excess of about 2-5 weight percentgenerally does not warrant the use of higher concentrations.

The adsorbent support which may be used in the practice of thisinvention can be any of the well known adsorbent materials generallyutilized as a catalyst support or carrier material. Preferred adsorbentmaterials include graphite and the various charcoals produced by thedestructive distillation of wood, peat, lignite, nutshells, bones, andother carbonaceous matter, and preferably such charcoals as have beenheat-treated or chemically treated or both, to form a highly porousparticle structure of increased adsorbent capacity, and generallydefined as activated carbon or charcoal. Said adsorbent materials alsoinclude the naturally occurring or synthetic zeolitic and molecularsieve materials generally and also the naturally occurring orsynthetically prepared refractory inorganic oxides such as alumina,silica, zirconia, thoria, boria, etc., or combinations thereof likesilica-alumina, silica-zirconia, alumina-zirconia, etc. Any particularsolid adsorbent material is selected with regard to its stability underconditions of its intended use. With regard to its intended use inaqueous systems, perhaps the most important property of the adsorbentsupport is its insolubility as well as complete unreactivity in aqueoussystems. Charcoal, and particularly activated charcoal, is preferredbecause of its capacity for metal chelates, and because of its stabilityunder treating conditions, at least at temperatures under about 150° C.At 150° C. and above charcoal tends to be oxidized and becomesunsuitable for use in a continuous process.

Our process oxidizes metal-complexed cyanides under acidic conditions,i.e., at a pH under 7. Our invention may be practiced at a pH as low asabout 0.5 and as high as about 6.5, but usually will be practiced in apH range between about 1 and about 6, more often in the range of 2-5.5,and most often in the range between about 3 and about 5. Where themetal-complexed cyanide-containing aqueous streams is initially acidicit may be oxidized directly without pH adjustment. In the more usualcase the aqueous stream must be acidified prior to oxidative treatment.In such cases the acid used for acidification does not have anysubstantial effect on the invention, and any suitable acid may bechosen. The mineral acids, especially sulfuric and hydrochloric adds,are the most common acids employed.

Although the invention as described may be practiced at a quite low pH,disadvantages accrue from materials corrosion and metal chelate leachingfrom the support and even chemical degradation of some supports. Hencethere are benefits from performing the reaction in a buffered solutionwhich tends to maintain the system at a moderate pH, e.g., above aboutpH 3. The nature of the buffer is quite immaterial so long as it isitself not oxidized under reaction conditions and does not chemicallyinterfere with cyanide oxidation. Suitable buffers may be based on,e.g., phosphates, acetates, carboxylates generally, borates, carbonates,and so forth. It may be well to mention that since both CO₂ and NH₄ ⁺are reaction products there will be substantial internal bufferingoccurring as the reaction proceeds. This is yet another benefit flowingfrom our invention. An ancillary benefit of our invention is that theproducts are virtually exclusively carbon dioxide and ammonium ion ornitrogen.

Although the process which is our invention can operate under ambientconditions of temperature and pressure, it is found that formetal-complexed cyanides the slow rate of reaction, which reflects inpart the low degree of dissociation of the complex to free cyanide,requires a temperature of at least 7° C., usually in excess of 100° C.,and often of at least 150° C. in order to achieve a commerciallydesirable rate of reaction. Usually a temperature greater than 250° C.is not necessary. If the reaction is conducted at 1 atmosphere pressure,one is limited to an upper temperature of about 95° C. for aqueoussystems because of the increased vapor pressure arising from water.Therefore, because it is usually necessary to conduct the oxidation attemperatures about 95° C. super-atmospheric pressures generally will benecessary. Other than maintaining the aqueous system in a liquid state,the operating pressure has no significant effect on the success orperformance of our invention.

The higher is the reaction temperature, the faster the cyanide oxidationwill proceed. Similarly, the higher the partial pressure of oxygen --assuming its use as the sole oxidant--the faster will the reactionproceed. Consequently there are some advantages to working at partialpressures of oxygen higher than 1 atm. and at as high a temperature aspossible under the reaction pressures employed. As a practical matter,it is believed that temperatures in excess of about 250° C. andpressures in excess of about 50 atmospheres will prove only marginallybeneficial and that no real economic benefit will accrue from practicingthe invention herein under more stringent conditions.

It is also possible to practice our invention using either a flowingoxygen-containing gas stream or by presaturating the feedstream withoxygen and then oxidizing the saturated feedstream. In the first variantthe reactants are in a two-phase system, and in the second variant thereactants are in a single-phase system. The variant where there is aflowing oxygen-containing gas stream presents the advantage that oxygenalways can be present in great excess, although not in solution with thecyanide. Accordingly, some phase transport problems may arise. In thevariant where all the oxygen is present in the feed stream oxygentransport is easier but the extent of cyanide oxidation, the rate ofcyanide oxidation, or both, may be limited by the concentration ofdissolved oxygen. Which variant is chosen is largely a matter of designchoice.

As previously mentioned, the preferred oxidizing agent is oxygen,whether from air or from an oxygen-enriched gas. Other oxidants also maybe used, in particular hydrogen peroxide and ozone, but these are notseen to be as generally convenient as the use of oxygen. Where thecyanide content of the aqueous stream is no more than about 15 ppm, onecan readily use air at atmospheric pressure as the source of oxygen, forunder these conditions the level of dissolved oxygen will be sufficientfor the concentration of cyanide present. On the other hand, one can goto higher pressures to effect higher concentrations of dissolved oxygen.However, we have found it more effective to continually bubble oxygenthrough the cyanide-laden aqueous stream in the reaction zone in orderto provide sufficient oxygen for oxidation of cyanide at levelsconsiderably higher than 15 ppm. Adequate dispersal of oxygen in theaqueous feedstock in contact with the metal chelate as catalyst is ofconsiderable importance, but since appropriate methods of dispersal arewell known in the art these will not be further discussed. Where aperoxide, such as hydrogen peroxide, is used as the oxidizing agent itcan be conveniently added to the feedstock in an mount adequate tocompletely oxidize the cyanide present.

Although it is believed that temperature, oxidant concentration, and pHare the most important variables in the practice of our invention, otherfactors such as residence time, metal-complexed cyanide concentration,and extent of dissociation of the complexed cyanide constitute otherprocess variables which the skilled worker will readily adapt to. Theprocess variables can be changed over a rather broad range to affect theamount of cyanide oxidized. No inviolate rules can be stated for thedegree of cyanide which should be removed; our previous comments showedno standard was applicable to all feedstocks and discharges. Onedesirable characteristic of our process is that removal of 90% of thecyanide is routine, removal of 95% is not difficult, and removal ofgreater than 98% is well within process capabilities.

The process of our invention can be practiced in a multiplicity ofmodes. Although practicing the invention using a water-insoluble metalchelate is anticipated to be the most widespread mode used, one canenvision circumstances where a water-soluble catalyst is preferred. Forexample, the aqueous stream may come from the mining industry andcontain a considerable amount of solids. Removal of the solids prior tooxidation of cyanide would lead to a solid mass containing substantialamounts of cyanide which itself might present serious disposal problems.In such a case it may be advantageous to use a water-soluble metalchelate to catalyze the oxidation of cyanide. It also should be clearthat propitious choice of the metal in the metal chelate needs to bemade in order to minimize contamination by the metal of the metalchelate when the aqueous stream is later disposed of.

As previously alluded to, in the vast majority of cases it is expectedthat a water-insoluble metal chelate will be used in order to effect aheterogeneous catalysis of cyanide oxidation. In such a mode it isadvantageous to impregnate the metal chelate on a water-insolublecarder, as described above, in order to effect as high a dispersal ofthe metal chelate as possible. One mode of oxidation would employ, or beanalogous to, a slurry reactor, where the water-insoluble metal chelate,preferably dispersed on a water-insoluble carrier, is suspended in theaqueous feedstock and reaction is carried out using this well mixedsuspension. Slurry reactions can be carried out either batchwise orcontinuously. In the continuous mode solids are removed from thefeedstock after oxidation of cyanide and mixed with and resuspended infresh feedstock passing into a slurry reactor.

However, it is contemplated that the process of our invention will bemost useful when practiced in a continuous mode using a packed bed ofthe metal thelate dispersed on a suitable support. The metal-complexedcyanide-laden acidic feedstock can be passed either upflow or downflow,and the oxygen passed either concurrently or countercurrently. In yetanother variation, suitable where the cyanide concentration of themetal-complexed cyanide is less than about 15 ppm, the feedstock can besaturated with oxygen prior to being contacted with the metal chelate inthe reaction zone. As discussed previously, the level of oxygendissolved in water is sufficient to oxidize up to about 15 pans permillion cyanide, which accounts for the operability of the lastdescribed embodiment.

Even though the continuous oxidation of cyanide using a packed bed of ametal chelate dispersed on a suitable support may be practiced in any ofthe aforementioned modes, it has been found that a cocurrent oxygen feedmay lead to oxygen-starved media and thereby may limit the amount ofcyanide which can be oxidized under a given set of experimentalconditions. Where this occurs one may operate a packed bed reactor in atrickle bed mode with countercurrent oxygen flow, that is, the aqueousfeedstock flows downward over the packed catalyst bed and the oxygen ispassed upward through the packed catalyst bed. It is anticipated that inthis mode it is feasible to satisfactorily oxidize cyanide atconcentrations at least as high as about 500 ppm when working at apressure of air (as the sole oxygen source) of 1 atmosphere and areaction temperature no more than about 95° C. It is expected thatsubstantially higher cyanide concentrations can be used at higherpartial pressures of oxygen and higher reaction temperatures. Especiallywhere higher partial pressures of oxygen (i.e., over about 0.2atmospheres) are used, or where oxygen addition is staged, cocurrentoxygen flow may provide adequate oxygen and may be preferred foreconomic reasons.

Other embodiments and variants will be apparent to the skilled worker,all of which are intended to be encompassed within and subsumed by ourinvention as claimed. The following examples merely illustrate severalaspects of this invention. The examples are not intended to beexhaustive nor to restrict our invention in any way, and in particularour invention is not to be thought of as being limited to the examplesthemselves.

EXAMPLES

A feedstock containing 411 ppm K₃ Fe(CN)₆, equivalent to 200 ppmcyanide, at pH 3.4, was passed downflow over a bed of 100 cc (32 g.) ofa catalyst consisting of cobalt phthalocyanine deposited on charcoal inan amount sufficient to contain 259 ppm cobalt. Air was passedcountercurrently at 0.08 cubic feet per hour, and the system wasoperated at 600 psig. How rate and temperature were varied, with thecyanide remaining analyzed at each set of conditions. Results aresummarized in Table 1. Little isocyanate was formed (<1 ppm) and cyanideconversion was virtually exclusively to CO₂ and NH₄ ⁺. It can be seenthat oxidation at 150° C. was quite effective in destroying cyanide.

                                      TABLE 1                                     __________________________________________________________________________    Oxidation of Cyanide from K.sub.3 Fe(CN).sub.6                                Hours on  FREE.sup.b CN--                                                                       COMPLEXED.sup.c CN--                                                                      CYANATE                                         Stream                                                                             LHSV.sup.a                                                                         (ppm)   (ppm)       (ppm)  °C.                               __________________________________________________________________________     0    0.45                                                                              2.3     70          1       50                                       12  "    1       70          1      50                                        18  "    1       72          1      50                                        24  "    1.5     66          1      50                                        30  "    1.7                 1      50                                        42  0.9  2.1                 1      50                                        54  "    3.2                 1      50                                        66  "    3       119         1      50                                        78  "    3.4     120         1      50                                        90  "    2.4                 1      50                                       102  "    2       140         1      50                                       114  1.8  1.8     140         1      50                                       126  "    4.2                 1      75                                       138  "    4.9     110         1      75                                       150  "    8.7     120         1      75                                       162  0.9  10                  1      75                                       174  "    9.6                 1      75                                       186  "    12      94          1      75                                       198   0.45                                                                              12      99          1      75                                       210  "    12      99          1      75                                       222  "    10      61          1      75                                       234  "    10      44          1      75                                       246  "    10      65          1      75                                       258  "    10                  1      100                                      270  "    12      36          1      100                                      282  0.9  18      46          1      100                                      294  "    23                  1      100                                      306  "    13                  1      100                                      318  "    15      96          1      100                                      330  "    17      100         1      100                                      342  "    19                  1      100                                      354  "    17                  1      150                                      366  0.3  1       130         1      150                                      378  "    1       14          1      150                                      390  "    1       14          1      150                                      402  "    1       14          1      150                                      414  "    1       14          1      150                                      __________________________________________________________________________     .sup.a Liquid hourly space velocity, in hr.sup.-1                             .sup.b Non-complexed cyanide remaining in solution                            .sup.c Cyanide in solution present as a metal complex                    

What is claimed is:
 1. A method of reducing the water-solublemetal-complexed cyanide concentration in an aqueous stream comprisingoxidizing the metal-complexed cyanide under acidic oxidation conditionswith an oxidizing agent selected from the group consisting of oxygen,ozone, and hydrogen peroxide in the presence of a catalyticallyeffective amount of a metal chelate, where said metal chelate isselected from the group consisting of metal compounds oftetrapyridinoporphyrazine, porphyrin, corrinoid materials, and thephthalocyanines, wherein the cyanide is oxidized to carbon dioxide,nitrogen, and ammonium ion.
 2. The method of claim 1 where the metalchelate is water soluble.
 3. The method of claim 1 where the metalthelate is water insoluble.
 4. The method of claim 1 where the metalchelate is water insoluble and supported on a water-insoluble carrier.5. The method of claim 4 where the carrier is selected from the groupconsisting of graphite, charcoal, zeolitic and molecular sieve materialsand refractory inorganic oxides.
 6. The method of claim 5 where thecarrier is charcoal.
 7. The method of claim 1 where the metal in themetal chelate is selected from the group of iron, copper, cobalt, andvanadium.
 8. The method of claim 7 where the metal chelate is a cobaltchelate.
 9. The method of claim 1 where the metal chelate is asulfonated cobalt phthalocyanine.
 10. The method of claim 1 whereoxidation conditions include a temperature from about 75° C. up to about250° C. and a total pressure from about 1 atmosphere up to about 50atmospheres.
 11. The method of claim 1 where the acidic oxidationconditions include a pH from 1 to a pH of about
 6. 12. The method ofclaim 1 where the acidic oxidation conditions include a pH from about 2to about 5.5.
 13. The method of claim 1 where the acidic oxidationconditions include a pH from about 3 up to about
 5. 14. The method ofclaim 1 where the metal-complexed cyanide concentration is reduced by atleast 90 percent.
 15. The method of claim 14 where the metal-complexedcyanide concentration is reduced by at least 95 percent.
 16. The methodof claim 15 where the metal-complexed cyanide concentration is reducedby at least 98 percent.
 17. The method of claim 1 where the metal of themetal-complexed cyanide is selected from the group consisting of iron,nickel, chromium, copper, cadmium, cobalt, manganese, tungsten,molybdenum, gold, and silver.
 18. The method of claim 1 where themetal-complexed cyanide is one which furnishes at 20° C. less than 1percent of the available cyanide as free cyanide.
 19. The method ofclaim 1 where the metal-complexed cyanide is one which furnishes at 20°C. less than 0.1 percent of the available cyanide as free cyanide.
 20. Amethod of reducing the cyanide concentration in a metal-complexedcyanide-containing aqueous stream by oxidizing the metal-complexedcyanide under acidic conditions with oxygen comprising flowing at acidicoxidation conditions the metal-complexed cyanide-containing aqueousstream through a bed of a catalytic composite, said composite comprisinga metal chelate supported on a carrier, flowing an oxygen-containing gasthrough said bed, and removing the effluent having a reducedmetal-complexed cyanide concentration, where said metal chelate isselected from the group consisting of metal compounds oftetrapyridinoporphyrazine, porphyrin, corrinoid materials, and thephthalocyanines, wherein the cyanide is oxidized to carbon dioxide,nitrogen, and ammonium ion.
 21. The method of claim 20 where the aqueousstream flows down through the bed.
 22. The method of claim 20 where thecarrier is selected from the group consisting of graphite, charcoal,zeolitic and molecular sieve materials, and refractory inorganic oxides.23. The method of claim 22 where the carrier is charcoal.
 24. The methodof claim 20 where the metal in the metal chelate is selected from thegroup of iron, copper, cobalt, and vanadium.
 25. The method of claim 20where the metal chelate is a cobalt chelate.
 26. The method of claim 20where the metal chelate is a sulfonated cobalt phthalocyanine.
 27. Themethod of claim 20 where the oxidation conditions include a temperaturefrom about 75° C. up to about 250° C. and a total pressure from about 1atmosphere up to about 50 atmospheres.
 28. The method of claim 20 wherethe acidic oxidation conditions include a pH from 1 to a pH of about 6.29. The method of claim 20 where the acidic oxidation conditions includea pH from about 2 to about 5.5.
 30. The method of claim 20 where theacidic oxidation conditions include a pH from about 3 up to about
 5. 31.The method of claim 20 where the cyanide concentration is reduced by atleast 90 percent.
 32. The method of claim 31 where the cyanideconcentration is reduced by at least 95 percent.
 33. The method of claim32 where the cyanide concentration is reduced by at least 98 percent.34. The method of claim 20 where the oxygen-containing gas flowscountercurrent to the aqueous stream.
 35. The method of claim 20 wherethe oxygen-containing gas flows cocurrent with the aqueous stream. 36.The method of claim 20 where the metal-complexed cyanide is one whichfurnishes at 20° C. less than 1% of the available cyanide as freecyanide.
 37. The method of claim 36 where the metal-complexed cyanide isone which furnishes at 20° C. less than 0.1% of the available cyanide asfree cyanide.